In recent years, photoconductive materials are actively studied, and some of them are used as photoelectric conversion elements for electrophotographic photoreceptors, solar batteries, image sensors and the like. As such photoconductive materials, inorganic materials have been mainly used up to the present. In electrophotographic photoreceptors, for example, there have been broadly used inorganic photoreceptors provided with a photosensitive layer which contains selenium, zinc oxide, or cadmium sulfide as the primary component.
However, these inorganic photoreceptors are not always satisfactory in characteristics of photosensitivity, thermal stability, moisture resistance and durability, which are essential for electrophotographic photoreceptors used in copying machines. For example, selenium is apt to lower its properties as an electrophotographic photoreceptor for its liability to crystallization by heat or stains such as finger spots.
Further, electrophotographic photoreceptors using cadmium sulfide are low in moisture resistance and durability, and electrophotographic photoreceptors using zinc oxide are insufficient in durability.
Moreover, in the growing importance of environmental preservation, electrophotographic photoreceptors of cadmium sulfide have a defect of requiring severe control in both manufacturing and handling because of its toxicity.
For improving such disadvantages of inorganic photoconductive materials, various organic photoconductive materials have come to attract much attention in the art, and a number of approaches are being made to utilize them in the photosensitive layer of electrophotographic photoreceptors. For example, Japanese Pat. Exam. Pub. No. 10496/1975 discloses an organic photoreceptor having a photosensitive layer containing polyvinylcarbazole and trinitrofluorenone. This photoreceptor, however, is not sufficient in sensitivity and durability. Taking notice of this, there are developed electrophotographic photoreceptors of function-separated type which assign the carrier generation function and the carrier transport function to different substances separately.
These function-separated electrophotographic photoreceptors allow materials to be selected appropriately from a broad range; therefore, objective properties are obtained relatively easily, and development of an organic photoreceptor of high sensitivity and excellent durability is expected.
There are proposed various organic materials as a carrier generation material or a carrier transport material in such function-separated electrophotographic photoreceptors. And the carrier generation substance has a particularly important function which controls basic properties of a photoreceptor. As such carrier generation substances, there have been practically used photoconductive substances such as polycyclic quinones represented by dibromoanthanthrone, pyrylium compounds and eutectic complexes thereof, squarium compounds, phthlocyanine compounds and azo compounds. Since the carrier generation substance is coated in the form of dispersion or solution of an organic solvent in general, a good dispersibility and high dispersion stability are required of a carrier generation substance to obtain good electrophotographic photoreceptors.
Further, a carrier generation substance having a high carrier generation efficiency is also necessary to impart a high sensitivity to an electrophotographic photoreceptor. In this connection, phthalocyanine compounds absorb much attention in recent years and are actively studied as a material to meet such necessity.
It is known that characteristics of the phthalocyanine compound including spectrum and photoconductivity vary according to types of central metals and crystal forms. For example, it is reported, in "Senryo to Yakuhin" (Dyes and Related Chemicals) by M. Sawata, 24 (6), p. 122 (1979), that copper phthalocynine has three crystal forms: .alpha., .beta., .gamma. and .epsilon., and that its electrophotographic characteristics vary depending upon the crystal forms.
Titanylphthalocyanine, which is particularly interested recently, is also reported to have four crystal forms of types A, B, C and Y. Titanylphthalocyanine of type A described in Japanese Pat. O.P.I. Pub. No. 67094/1987, type B in Japanese Pat. O.P.I. Pub. No. 239248/1986 and type C in Japanese Pat. O.P.I. Pub. No. 256865/1987 are still insufficient in electrophotographic sensitivity and durability.
Titanylphthalocyanine of type Y made known recently in "Japan Hardcopy '89" by Kinoshita et al., EP 26 (1989) has a high sensitivity; for utilizing its high characteristics practically by making its dispersion stably and finely, it is essential to develop a new technique to prepare its dispersion.